This invention relates to preparing gray or blue aromatic polyesters, preferably the reaction product of an aromatic dicarboxylic acid and bisphenol, through the addition of cobalt to the reaction system. Aromatic polyesters by themselves, such as formed by reacting bisphenol-A and terephthalate tend to have a yellowish tint. The addition of cobalt to the reaction system will produce a color of gray, blue, or even purple, depending on the level of cobalt added to the reaction system.
Color enhancement of polyesters is known in the prior art. French patent 1,287,519 discloses new polyesters made of phthalic acids, carbonic acids, and bisphenol. This patent discloses that it is difficult to get a desired coloration of these polyesters. Swiss patent 482,752, also Chemical Abstracts Vol. 72, 90977n, discloses the preparation of polyesters, copolyesters, or copolyether esters by using cobalt ethylene glycolate and/or lead ethylene glycolate. The product is a practically white, light-blue polyester. Japanese patent 50-82801, also Chemical Abstracts Vol. 86, 156232p, discloses a system using a lead benzoate or similar catalyst to make a bisphenol-A-diphenyl terephthalate-neopentyl glycol copolymer or similar polyester. These disclosures do not, however, illustrate the use of cobalt in order to obtain a broader range of color enhancement in the blue-to-gray area.
A persistent problem in the preparation of linear aromatic polyesters from reaction of a bisphenol and an aromatic carboxylic acid has been the undesirable high color in the product, i.e., the tendency of the polyester to develop an undesirable yellow color during the polyesterification. The color is attributed to thermal decomposition of the product polyester and/or the reactants during the reaction which is carried out at high temperatures ranging up to 400.degree. C., or higher, without the use of a diluent.